Stents are scaffolds which are positioned in diseased vessel segments to support the vessel walls. Stents are used in angioplasty to repair and reconstruct blood vessels. Placement of a stent in the affected arterial segment prevents elastic recoil and closing of the artery. Stents also prevent local dissection of the artery along the medial layer of the artery. Stents may be used inside the lumen of any physiological space, such as an artery, vein, bile duct, urinary tract, alimentary tract, tracheobronchial tree, cerebral aqueduct or genitourinary system. Stents may also be placed inside the lumen of human as well as non-human animals.
In general there are two types of stents: radially, self-expanding and radially, balloon-expandable. The balloon-expandable stent is placed in a diseased segment of a vessel by inserting an unexpanded stent into the affected area within the vessel The stent is expanded by positioning a balloon inside the stent and inflating the balloon to expand the stent. Inflation remodels the arterial plaque and secures the stent within the affected vessel. One problem with balloon stents is that the inside diameter of the stent may become smaller over time if the stent lacks expanding resilience. The result of this lack of resilience is that the stent collapses due to the natural elastic recoil of the blood vessel.
In contrast, a self-expanding stent is capable of expanding by itself. There are many different designs of self-expanding stents, including, coil (spiral), circular, cylinder, roll, stepped pipe, high-order coil, cage or mesh. Self-expanding stents are formed from super-elastic metal. See, for example, U.S. Pat. No. 6,013,854 to Moriuchi. The self-expanding stent is placed in the vessel by inserting the stent in a compressed state into the affected region, e.g., an area of stenosis. Once the compressive force is removed, the stent expands to fill the lumen of the vessel. The stent may be compressed using a tube that has a smaller outside diameter than the inner diameter of the affected vessel region. When the stent is released from confinement in the tube, the stent expands to resume its original shape and becomes securely fixed inside the vessel against the vessel wall.
Each of the various stent designs that have been used with self-expanding stents has certain functional problems. For example, a stent formed in the shape of a simple circular cylinder does not compress easily. Consequently, insertion of the stent into the affected region of a vessel may be very difficult.
One approach of the prior art stent designs to overcome this problem is to provide a stent formed by zigzag elements as disclosed in U.S. Pat. No. 5,562,697 to Christiansen. A stent formed from a zigzag pattern has flexibility in the axial direction to facilitate delivery of the stent, however, this type of stent often lacks sufficient radial strength to maintain patentcy of the vessel after elastic recoil.
In order to provide increased radial strength of the zigzag design, the zigzag elements may be connected with connection elements. U.S. Pat. No. 6,042,597 to Kveen et al. describes a balloon expandable stent formed by a continuous helical element having undulating portions which form peaks and troughs where all of the peaks of adjacent undulating portions are connected by curvilinear elements. Connection elements between each adjacent undulating portion may impair flexibility of the stent.
Another approach is to provide a plurality of interconnecting cells which are in the shape of a diamond or rhomboid as in U.S. Pat. No. 6,063,113 to Karteladze et al. or U.S. Pat. No. 6,013,584 to Moriuchi. This type of stent has cells which rigidly interlock. Consequently, these types of stents have a comparatively high degree of rigidity and do not bend to accommodate changes in vessel shape.
It will be appreciated that in spite of these disclosures, there is still a great need for a self-expanding stent that overcomes the deficiencies of the prior art stents. Accordingly, the present invention provides a geometric design for a stent that has both a high degree of flexibility and significant radial strength. The design of this stent also allows it to be inserted into small diameter vessels. The stent is further able to respond dynamically to changes in blood pressure.